Catheters are being used increasingly as a means for delivering diagnostic or therapeutic agents to internal target sites that can be accessed through the circulatory system. For example, in angiography, catheters are designed to deliver a radio-opaque agent to a target site within a blood vessel, to allow radiographic viewing of the vessel and blood flow characteristics near the release site. For the treatment of localized disease, such as solid tumors, catheters allow a therapeutic agent to be delivered to the target site at a relatively high concentration, with minimum overall side effects. Methods for producing localized vaso-occlusion in target tissue regions, by catheter injection of a vaso-occlusive agent, have also been described (co-owned U.S. patent application for "Hyperthermic Treatment of Tumors", Ser. No. 751,605, filed 2 July 1985).
Often the target site which one wishes to access by catheter is buried within a soft tissue, such as brain or liver, and can be reached only by a tortuous route through small vessels or ducts--typically less than about 3 mm lumen diameter--in the tissue. The difficulty in accessing such regions is that the catheter must be quite flexible, in order to follow the tortuous path into the tissue, and at the same time, stiff enough to allow the distal end of the catheter to be manipulated from an external access site, which may be as much as a meter or more from the tissue site.
Heretofore, two general methods for accessing such tortuous-path regions have been devised. The first method employs a highly flexible catheter having a dilated or dilatable distal end. A major limitation of this method is that the catheter will travel in the path of highest blood flow rate, so many target sites with low blood flow rates cannot be accessed.
In the second prior art method, a torqueable guide wire having a distal bend is guided, by alternately rotating and advancing the wire, to the target site. With the wire in place, a thin-walled catheter is then advanced along the wire until the distal catheter end is positioned at the target site. Once the catheter is advanced, the guide wire may be withdrawn to allow fluid delivery or withdrawal through the catheter. An important advantage of this method is the ability to control the location of the catheter along a vascular pathway.
Several types of guide wires for use in catheter placement have been proposed. The simplest type of wire is a preferred diameter of between about 8-40 mils (thousandths of an inch). The distal end of the wire may be provided with a bent tip which can be oriented, by means of guide structure at the proximal end, to guide the wire along a selected vascular path. Ideally, torque transmission should be controlled, such that a selected wire rotation at the wire's proximal end produces a corresponding rotation of the distal end. Because of their greater flexibility, smaller diameter wires, e.g., having diameters of between about 8-18 mils, may be required for accessing small-vessel and/or tortuous-path regions. However, if the wire is too thin along its entire length, it may be difficult to transmit torque in a controlled manner along the entire wire length. Further, the wire may buckle with axial movement due to low column strength.
Constant-diameter guide wires having a wire core encased in a flexible polymer tubing have also been proposed. The flexible tubing acts to increase the column strength of the wire core without significantly reducing overall flexibility. As a result, the problem of wire buckling, especially in small-diameter wires, is lessened. Biocompatible polymers, such as TEFLON.RTM., polyolefins, and polyurethane have been suitable.
More recently, guide wires which have multiple variable-thickness steps along the wire length have been proposed. Wires of this type have the advantage that the proximal end region, where greater torsional strength is required, have relatively large diameters--e.g., between about 20-40 mils, and the distal end region, where greater flexibility is required, have progressively smaller diameters. Typically, a wire of this type will have different diameter segments extending collectively over an approximately 25-60 cm distal portion of the wire, and a short (typically 1-3 cm) tapered transition zone across each step. The tapered zones are typically formed by centerless grinding in which the wire is placed between two counter-rotating grinding wheels whose confronting grinding surfaces are angled slightly to produce the desired taper over the width of the wheels.
If the tapered transition is relatively steep and/or transition occurs in a region where a sharp vessel bend is encountered, the wire may bend sharply in the step (transition) zone, due to the differential bending modulus at the transition zone. If the catheter on the wire has already been advanced past the point of the bend, the catheter may deform at the wire bend, making further catheter advance along the wire difficult or impossible. Further, torqueability in the wire is reduced at the region of a sharp bend, since torque tends to be transmitted through the angle of the bend, rather than along the axis of the wire.
Guide wires having extended sections of continuous taper have also been disclosed. The long tapered regions have less tendency to undergo irreversible bending than relatively short tapered wire sections. However, problems of wire buckling and difficulty in sliding the wire within the catheter in a tortuous path limit the ability of the wire and catheter to reach deep tissue sites.
The problems of advancing a catheter along a guide wire in a small-lumen tortuous tissue pathway are also due to limitations in prior art catheter construction. If the catheter is relatively rigid, it cannot track over the final distal portion of the wire in the tortuous path region, because catheter advancement buckles the wire in a narrow turn, or because catheter advancement pulls the wire out of the distal vessels. On the other hand, catheters having more flexible shafts, such as those used in balloon flow-directed devices, lack the column-strength in the catheter's proximal section to be advanced over the guide wire without buckling.